The present invention relates to a method for improving the performance of thin film organic circuits having a readily manufacturable structure in which an organic semiconducting material is formed on the surface of patterned electrodes.
Organic thin films (OFTs) have attracted considerable research interest due to their potential use as a replacement for other, more expensive semiconducting materials. Many organic materials, such as pentacene and hexithiopene, have performance properties that are competitive with amorphous silicon. Other superior characteristics, such as mechanical flexibility and the availability of room temperature deposition processes, make the organic materials possible replacements in low cost applications. These characteristics also make organic thin films a suitable candidate for use in low speed logic and radio frequency applications such as an active tag or a smart card.
The usual form of deposition of structures containing organic thin films is known in the art as an inverted thin film transistor structure. A conductive gate and an insulating layer are first formed on a suitable substrate. The gate is often a patterned or blanket metal or conductive diffusion layer. The insulator is typically SiO2 or an organic material such as polyimide. There are two major methods known in the art for defining the source and drain electrodes in such a structure: The first method comprises laying down the organic semiconductor, i.e. organic thin film, and then depositing the source/drain electrodes through a shadow mask. In the other method, the source and drain electrodes are deposited and patterned using any method, and then the organic semiconductor is deposited. The organic semiconductor may be unpatterned or patterned using, for example, a shadow mask.
There is a significant advantage in using a process in which the metal for the source and drain electrodes is laid down first. Namely, the metal may be deposited and patterned using any of several processes including photolithography and stamp etching. Once deposited onto the organic semiconducting material, metals are difficult to pattern since semiconducting organic layers are generally very sensitive to exposure to processing chemicals. The major disadvantage to patterning the source/drain electrodes first and then depositing the organic semiconductor is that the organic semiconductor exhibits inferior performance when deposited onto preexisting source/drain electrodes.
Experiments have demonstrated a significant performance penalty when the organic semiconductor is directly deposited onto preexisting source/drain electrodes relative to identical transistor geometry formed by deposition of the metal on top of the organic semiconductor material. This is believed to be caused by the less favorable structure of the organic thin film formed when deposited onto the metal. When the organic semiconductor is deposited onto a dielectric substrate, more self-assembly into larger grains is seen. This leads to a higher performance semiconductor.
There is no known recognition of this problem in the literature, and all reports of high performance organic devices use the top electrode geometry. Other techniques reported in the prior art for increasing the performance of organic thin film semiconductors use high temperatures (See, T. N. Jackson, et al. xe2x80x9cStacked Pentacene Layer Organic Thin-Film Transistors with Improved Characteristicsxe2x80x9d, IEEE Electron Device Letters, Vol. 18, No. 12, p. 606, December 1997), or use self-assembled monolayers on the gate insulator. These prior art techniques do not, however, address the problem of the metal contacting the organic thin film semiconductor.
The deposition processes used to date in the prior art thus suffer from the following disadvantages:
(i) Top electrode deposition of metal onto organic thin films is difficult to pattern lithographically.
(ii) Top electrode depositions generally require the use of shadow masks which are fragile; can only pattern small areas; are of lower resolution than the lithographic processes used to create them; and the shadow masks must be cleaned. Additionally, the edges of films defined by shadow masks are often irregular due to adhesion to the mask.
(iii) Bottom electrode depositions have inferior performance to top electrode deposition.
(iv) Use of an elevated temperature to improve performance restricts the type of substrates, which may be used to form the device.
(v) Use of an alignment layer on the substrate does not increase the ordering on the electrodes or at the electrode/semiconductor interface, and is not effective in increasing the grain size in small channels.
(vi) Use of a self-assembled monolayer on the substrate improves performance of some devices, but restricts-the choice of substrates to materials compatible with such layers (namely, SiO2 layers) and does not solve the performance penalty associated with the deposition of organic materials on metals.
In view of the above drawbacks in prior art processes of producing high performance structures containing organic semiconducting thin films, there is a need for developing a method in which the organic thin films can be formed on top of the source/drain electrodes without inducing a performance penalty.
One object of the present invention is to provide a process for producing high performance organic thin film transistors in which the source and drain electrodes are deposited and patterned before the organic semiconducting material is deposited.
Another object of the present invention is to provide a more readily manufacturable process for higher performance organic field effect transistors that can be used to create large area circuits.
A further object of the present invention is to provide a process for increasing the performance of organic field effect transistors that is independent of the deposition and patterning method of both the organic semiconductor and the source and drain metals.
Other objects of the present invention include:
(I) To provide a process which operates on two common metals that are compatible with organic semiconductors, namely gold and platinum.
(II) To provide a process which operates at room temperature.
(III) To provide a process which operates independently of the layers underneath the top surface of the metal used to form the electrodes, which may be serving as a more easily patterned seed electroplating layer, an adhesion layer or may serve some other purpose.
(IV) To provide a process that operates independently of the substrate used permitting, for example, the use of organic substrates.
(V) To provide a process that improves the performance of the devices at all scales, including very short channel lengths.
These and other objects and advantages can be achieved by utilizing the present method which includes a step of treating exposed surfaces of source and drain electrodes of a transistor structure with a thiol compound under conditions effective in forming a self-assembled monolayer of said thiol compound on said source and drain electrodes. It is noted that the thiol treatment step of the present invention is carried out before the organic semiconductor material is formed.
Specifically, the method of the present invention comprises the steps of:
(a) forming a transistor structure comprising at least patterned source and drain electrodes; and
(b) treating the patterned source and drain electrodes with a thiol compound having the formula:
RSH 
wherein R is a linear or branched, substituted or unsubstituted, alkyl, alkenyl, cycloalkyl or aromatic containing from about 6 to about 25 carbon atoms under conditions that are effective in forming a self-assembled monolayer of said thiol compound on said electrodes. It is again emphasized that the transistor structure of step (a) above does not include the-presence of an organic semiconducting thin film material. Instead, the organic semiconducting thin film material is formed in the structure only after step (b) is performed.
Another aspect of the present invention relates to an organic thin film transistor that comprises at least bottom source/drain electrodes having an organic semiconductor formed over said bottom source/drain electrodes, wherein said bottom source/drain electrodes have a self-assembled monolayer formed thereon, said self-assembled monolayer comprising a thiol compound having the formula:
RSH 
wherein R is a linear or branched, substituted or unsubstituted, alkyl, alkenyl, cycloalkyl or aromatic containing from about 6 to about 25 carbon atoms.
Single gate transistor structures and dual gate structures are contemplated. Thus, the present invention contemplates transistor structures wherein the gate region (gate insulator and gate) is formed below the organic semiconductor, above the organic semiconductor or both above and below the organic semiconductor.